This invention is directed to an improved container closure plug and more specifically a threaded plug having a unique gasket retaining feature.
In the shipping container industry it is most common to manufacture drums, pails and the like with one or more dispensing and/or fill openings. Such openings have an internal helical screw thread which merges into an unthreaded gasket sealing area. An externally threaded closure plug carrying an annular sealing gasket is threadedly engaged within the opening. Upon tightening the plug, the plug gasket is drawn down tightly against the unthreaded gasket sealing area to provide an effective sealing of the container. While many millions of drums and other containers have been sealed in this fashion, an occasional problem arises which this invention addresses. That problem has not only to do with sealing but also with unscrewing of the plug from the container opening. A critical relationship exists between the plug and its gasket on the one hand and the merger of the container opening thread into the opening gasket seat area on the other. This relationship is such that in practice upon applying the necessary torque to seal the plug in the container opening, the plug gasket becomes wedged against the run-out of the opening internal thread. When the plug is subsequently unscrewed it is an all to common occurrence that this gasket wedging action against the opening thread tightly grips the plug gasket. As the unscrewing action continues a tenuous condition ensues. Most notably as the plug travels out of the opening the gasket can remain ensnared on the internal opening thread so that the gasket gets pulled off of its gasket seat position on the plug. Continued rotation of the plug becomes extremely difficult as the gasket jams between the mating threads. Moreover, the gasket itself becomes severely damaged due to this shredding action which further diminishes the utility of the closure.
Another negative aspect of this gasket gripping condition is the tendency of the gasket to loop out of its gasket seat during torquing of the plug. This problem occurs when the plug gasket movement around the unthreaded gasket seat of the opening becomes obstructed causing the gasket to bunch up and form a loop protruding from the edge of the opening. One way for such obstruction to occur is when the plug gasket again becomes ensnared on the opening thread run-out as the plug now is travelling into the opening. The resultant “looping” condition will most certainly create a leakage path and quite likely impart serious damage to the gasket. The commonality between these “gripping” and “looping” problems is, of course, the ability to retain the plug gasket in place on the plug gasket seat where it belongs and thus prevent it from being dragged onto the thread interengagement.
The prior art has recognized the desirability generally of providing some positive mechanical retaining means for keeping the plug gasket in place. For example, U.S. Pat. No. 5,211,304 to Stolzman discloses a plug construction wherein the gasket is securely retained on the undersurface of the plug rim with a mechanical interlocking arrangement. This arrangement, however, places the gasket radially outwardly of the plug thread and has no relationship whatsoever to any kind of gasket “gripping” or “looping” problem. Another prior art example is found in U.S. Pat. No. 2,906,429 to Marchyn which recognizes the “looping” problems but fails to provide a completely satisfactory solution. In this patent the plug has a special thread formation where the thread borders the gasket seat. Specifically the plug has a diminishing thread that continues around the lower edge of the gasket seat and acts as a partial barrier above the normal plug thread. This modified thread configuration, however, falls short of providing adequate protection against the “gripping” phenomenon as will be clearly seen hereinafter.